1. Field of the Invention
The invention relates to a method and arrangement for watermarking an information signal, for example, an audio or video signal. The invention also relates to a method and arrangement for detecting a watermark in such an information signal.
2. Description of the Related Art
A known method of watermarking a video signal is disclosed in International Patent Application WO-A-99/45705, corresponding to U.S. patent application Ser. No. 09/423,273, filed Mar. 2, 1999. In this method, a watermark pattern is added to the video signal. A watermark detector correlates the same pattern with the suspect signal. If the correlation exceeds a given threshold, the pattern is said to be present. The presence or absence of the pattern represents a single bit of information. The embedded watermark may also carry a multi-bit payload. In the system disclosed in WO-A-99/45705, the payload is represented by a combination of one or more basic patterns and spatially shifted versions thereof. The payload is encoded into the respective shift vectors. The watermark detector correlates each basic pattern with the suspect signal, and determines the spatial positions of the basic patterns with respect to each other. The detector further checks whether said positions constitute a valid payload.
The process of correlating watermark patterns with the suspect signal requires the watermark detector to have locally stored versions of said patterns. In view thereof, it is desired that the watermarking system employs only a few different patterns. The patterns being used are kept secret to the outside world. However, even without knowledge of the patterns, a hacker can compromise the system if he has the relevant embedder at his disposal. He/she may feed an arbitrary input signal to said embedder and subtract the signal from its watermarked version. The difference signal thus obtained resembles the watermark of any other watermarked signal, depending on the perception model used in the watermark embedder at hand. If the difference signal is combined with (e.g., added to or subtracted from) a watermarked signal, the embedded watermark will substantially be cancelled or at least no longer represent a valid payload. In either case, the embedded watermark has been made ineffective.